Co2 reduction catalyst, co2 reduction electrode and co2 reduction device

ABSTRACT

The present embodiments provide a CO 2  reduction catalyst which is used for a reduction reaction of carbon dioxide and shows a high efficiency in water, and a CO 2  reduction electrode and a CO 2  reduction device, which contain the CO 2  reduction catalyst. This catalyst contains a conductive material and a porphyrin complex which has a specific structure and is insoluble in water. The porphyrin complex is insoluble in water because it contains only a small number of hydrophilic groups in its structure. The CO 2  reduction electrode and the CO 2  reduction device contain this catalyst.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2016-171975, filed on Sep. 2, 2016, the entire contents of which are incorporated herein by reference.

FIELD Embodiments of the present invention relate to a CO₂ reduction catalyst, a CO₂ reduction electrode and a CO₂ reduction device. BACKGROUND

In recent years, from the standpoints of energy problems and environmental issues, artificial photosynthesis technology which mimics plant photosynthesis and electrochemically converts solar energy into chemical energy has been developed. As compared to converting sunlight into electricity and storing the electricity in a storage battery, it is advantageous to convert solar energy into chemical energy, entrap the chemical energy in a chemical substance (high-energy substance) and store the chemical substance in a cylinder or a tank in that the energy storage cost can be reduced and storage loss is smaller.

Until now, technologies for extracting hydrogen, which is a high-energy substance (primarily a chemical fuel), from water have been gradually established. As a technology which utilizes light energy, photoelectrochemical reaction devices that comprise a laminate (e.g., a silicon solar cell) in which a photovoltaic layer is sandwiched by a pair of electrodes have been studied. On the electrode in the light-irradiated side of such a device, a reaction which oxidizes water (2H₂O) with light energy and yields oxygen (O₂) and hydrogen ions (4H⁺) takes place. On the other electrode, a reaction which utilizes the hydrogen ions (4H⁺) generated on the electrode in the light-irradiated side and an electric potential (e⁻) generated in the photovoltaic layer to produce chemical substances such as hydrogen (2H₂) takes place. Further, photoelectrochemical reaction devices in which silicon solar cells are laminated are also known. However, in these methods, although sunlight is converted into chemical energy with high efficiency, it is not easy to store and transport the thus produced hydrogen. Considering the energy problems and environmental issues, it is preferred to allow an easily storable and transportable carbon compound other than hydrogen to entrap the chemical energy.

Incidentally, a technology which highly efficiently converts CO₂ existing in a large amount in the atmosphere or the like into a chemical substance or the like useful as a chemical fuel has not been established. Still, at the laboratory level, photoelectrochemical reaction devices utilizing light energy have been examined. For example, there is known a device of a two-electrode system which comprises an electrode containing a reduction catalyst for reduction of carbon dioxide (CO₂) and an electrode containing an oxidation catalyst for oxidation of water (H₂O), wherein the electrodes are immersed in CO₂-dissolved water. In this device, the electrodes are electrically connected via an electric wire or the like. On the electrode containing an oxidation catalyst, as in the case of extracting hydrogen from water, H₂O is oxidized by light energy and oxygen (½O₂) is thereby produced and, at the same time, an electric potential is generated. The electrode containing a reduction catalyst reduces CO₂ by acquiring the electric potential from the electrode eliciting the oxidation reaction, whereby formic acid (HCOOH) and the like are produced. There are several reports on such a device.

According to the investigation by the present inventors, it is also known to utilize a porphyrin complex in these technologies. In such a known technology, CO₂ is reduced to CO by utilizing a porphyrin complex dissolved in an organic solvent as a CO₂ reduction catalyst. However, in this technology, although the complex is required to be dissolved or dispersed, since porphyrin is insoluble in water and it is thus difficult to dissolve porphyrin in water, the reaction hardly takes place in water. In addition, since the reaction is not an electrode reaction, a sacrificial reagent is necessary.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a CO₂ reduction electrode according to one embodiment; and

FIG. 2 is a schematic view of a CO₂ reduction device according to one embodiment.

DETAILED DESCRIPTION

Embodiments will now be explained with reference to the accompanying drawings.

The CO₂ reduction catalyst according to the present embodiment comprises:

a conductive material; and

a porphyrin complex which is carried on the conductive material and represented by the following Formula (A):

(wherein, Rs each represent a group selected from the group consisting of hydrogen, hydrocarbon groups having 1 to 12 carbon atoms, a hydroxyl group, an amino group, a carboxy group, a sulfo group, a mercapto group and a formyl group and are optionally the same or different, and adjacent Rs are optionally bound with each other via a hydrocarbon chain having 1 to 12 carbon atoms to form a cyclic structure;

M represents a (2+n)-valent metal ion, wherein n is a number of 0 or larger;

X represents an n-valent anion; and

the total number of hydroxyl groups, amino groups, sulfo groups and mercapto groups that are contained in one molecule of a porphyrin complex is 10 or less).

The method of reducing carbon dioxide according to the present embodiment comprises the steps of:

bringing a CO₂ reduction electrode comprising the above-described CO₂ reduction catalyst into contact with an electrolyte solution; and

introducing carbon dioxide to the electrolyte solution and reducing the thus introduced carbon dioxide by the above-described electrode.

Further, the CO₂ reduction device according to the present embodiment comprises:

an oxidation electrode;

a CO₂ reduction electrode comprising the above-described CO₂ reduction catalyst; and

a power supply element connected to the oxidation electrode and the CO₂ reduction electrode.

The “CO₂ reduction catalyst” (hereinafter, for convenience, may be simply referred to as “catalyst”) according to the present embodiment has a function of inducing the generation of a carbon compound by a CO₂ reduction reaction. In the present embodiment, the term “CO₂ reduction catalyst” does not mean a compound which has a function of inducing or promoting a CO₂ reduction reaction but means such a compound which is integrated with a conductive carrier carrying the compound.

The catalyst according to the present embodiment comprises a conductive material and a porphyrin complex having a specific structure. In this catalyst, since the porphyrin complex serving as the active center of the reaction is fixed by the conductive material and thus unlikely to elute into an electrolyte, a sacrificial reagent or the like is not required for reduction reaction.

In the present embodiment, the porphyrin complex is a material which has a function of inducing or promoting a CO₂ reduction reaction by lowering the activation energy for the reduction of CO₂. In other words, the porphyrin complex is a material which reduces an overvoltage occurring during the generation of a carbon compound by a CO₂ reduction reaction. In the present embodiment, as such a material, a porphyrin complex represented by the following Formula (A) is used:

(wherein, Rs each represent a group selected from the group consisting of hydrogen, hydrocarbon groups having 1 to 12 carbon atoms, a hydroxyl group (—OH), an amino group (—NH₂), a carboxy group (—C(═O)OH), a sulfo group (—SO₃H), a mercapto group (—SH) and a formyl group (—C(═O)H), and are optionally the same or different, and adjacent Rs are optionally bound with each other via a hydrocarbon chain having 1 to 12 carbon atoms to form a cyclic structure;

M represents a (2+n)-valent metal ion, wherein n is a number of 0 or larger;

X represents an n-valent anion; and

the total number of hydroxyl groups, amino groups, sulfo groups and mercapto groups that are contained in one molecule of a porphyrin complex is 10 or less).

The above-described hydrocarbon groups may be saturated or unsaturated and linear or branched. In addition, the hydrocarbon groups may have a cyclic structure. Specific examples of such hydrocarbon groups include a methyl group, an ethyl group, an n-propyl group, an i-propyl group, an n-butyl group, an n-pentyl group, an n-hexyl group, an octyl group, a cyclohexyl group, a cyclooctyl group, a cyclohexylmethyl group, a phenyl group, a tolyl group, a naphthyl group and a benzyl group. These hydrocarbon groups may further have a substituent(s) such as a hydroxyl group, an amino group and/or a carboxyl group. Moreover, adjacent Rs may be bound with each other via a hydrocarbon chain to form a cyclic structure. In this case, the hydrocarbon chain may be saturated or unsaturated. Examples of such a structure include a phthalocyanine structure in which the Rs at 2- and 3-positions, 7- and 8-positions, 12- and 13-positions and 17- and 18-positions are bound via unsaturated hydrocarbon chains and form aromatic rings.

The above-described amino group may be substituted with one or two hydrocarbon groups having 1 to 12 carbon atoms. Specific examples of such an amino group include a methylamino group, a dimethylamino group and a methylethylamino group.

The above-described metal ion is an ion of an element selected from the group consisting of Groups 8, 9 and 10 elements, among which Fe, Co, Ni, Ru, Rh, Pd, Mn and Co ions are preferred, a Fe ion is more preferred, and a trivalent Fe ion is particularly preferred. Since these metal ions can each take a plurality of valences, carbon dioxide coordinated with these metals is reduced.

The above-described anion neutralizes an electric charge when the valence of the metal ion is higher than 2. Accordingly, the anion does not exist when the metal ion is divalent. When the metal ion is trivalent, n is 1 and the metal ion M is bound with a single monovalent anion X. When the metal ion M is tetravalent, n is 2 and the metal ion is bound with two monovalent anions X or a single divalent anion X. Examples of the anion X include halogen ions such as chlorine and fluorine ions, a hydroxide ion, a bicarbonate ion, a carbonate ion, a bisulfate ion and a sulfate ion.

It is preferred that the porphyrin complex of the present embodiment does not dissolve in water. As described below, the reason for this is to fix the porphyrin complex on a carrier so as to not only obtain the catalytic effect of the porphyrin complex on the electrode surface but also inhibit elution of the porphyrin complex into a reaction medium such as an electrolyte solution. Therefore, it is preferred that the amount of a hydrophilic group(s) contained in the porphyrin complex used in the present embodiment be small. Specifically, the total number of hydroxyl groups, amino groups, sulfo groups and mercapto groups that are contained in one molecule of the porphyrin complex is required to be 10 or less, preferably 8 or less, more preferably 6 or less, most preferably 0. It is noted here that these hydrophilic groups are contained in a porphyrin ligand and an anion binding to the metal ion M is not included in these hydrophilic groups.

Among such porphyrin complexes, those having aromatic groups at the 5-, 10-, 15- and 20-positions and those having saturated hydrocarbon groups at the 2-, 3-, 7-, 8-, 12-, 13-, 17- and 18-positions are preferred because of the ease of synthesis and the availability. Among these porphyrin complexes, ones represented by the following Formula (A-1) or (A-2) are particularly preferred.

In these Formulae, R's each represent a group selected from the group consisting of hydrogen, hydrocarbon groups having 1 to 12 carbon atoms and are optionally the same or different, and adjacent R's are optionally bound with each other via a hydrocarbon chain having 1 to 12 carbon atoms to form a cyclic structure; and the above-described hydrocarbon groups and hydrocarbon chain are optionally substituted with a hydroxyl group, an amino group, a carboxy group, a sulfo group, a mercapto group or a formyl group, with a proviso that the total number of hydroxyl groups, amino groups, sulfo groups and mercapto groups that are contained in one molecule of the above-described porphyrin complex is 10 or less, preferably 8 or less, more preferably 6 or less, most preferably 0.

Examples of such a porphyrin complex include 5,10,15,20-tetraphenyl-21H,23H-porphyrin iron (III) chloride and 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin iron (III) chloride.

In the present embodiment, a porphyrin complex layer may contain two or more types of the above-described porphyrin complexes and may additionally contain a different metal, a metal compound, a metal complex, an organic compound or the like which exhibits a CO₂-reducing action.

As described above, the porphyrin complex layer has a function of inducing the generation of a carbon compound by a CO₂ reduction reaction. The carbon compound generated by the reduction reaction varies depending on the type and the like of the porphyrin complex. Examples of the carbon compound include carbon monoxide (CO), formic acid (HCOOH), methane (CH₄), methanol (CH₃OH), ethane (C₂H₆), ^(ethylene) (C₂H₄), ethanol (C₂H₅OH), formaldehyde (HCHO), acetaldehyde (CH₃CHO), acetic acid (CH₃COOH), ethylene glycol (HOCH₂CH₂OH), 1-propanol (CH₃CH₂CH₂OH) and isopropanol (CH₃CHOHCH₃).

Such a porphyrin complex can be obtained at a lower cost than conventional noble metals; therefore, there is an advantage that the catalyst can be produced inexpensively.

The conductive material used in combination with the porphyrin complex is not particularly restricted as long as it is capable of coming into contact with the porphyrin complex such that an electrical continuity can be established therebetween, and the conductive material is preferably one which contains a carbon material such as a carbon black, carbon nanotubes (CNT), graphene or fullerene.

A variety of carbon blacks that differ in particle size, particle shape and particle structure are known, and any of these carbon blacks can be used. Examples thereof include

Ketjen Black, Acetylene Black, Norit and Vulcan (all of which are registered trademarks). Alternatively, a metal material can be used as the conductive material. Examples of a metal material that can be used include metals such as Au, Ag, Cu, Al, Pt, Ni, Zn, Sn, Bi and Pd, and alloy materials containing a plurality of these metals, such as SUS. Moreover, for example, a translucent metal oxide such as ITO (indium tin oxide), ZnO (zinc oxide), FTO (fluorine-containing tin oxide: fluorine-doped tin oxide), AZO (aluminum-containing zinc oxide: aluminum-doped zinc oxide) or ATO (antimony-containing tin oxide: antimony-doped tin oxide) may also be used as the conductive material.

These various conductive materials can be used in a combination of two or more thereof.

The conductive material may be in the form of a plate, a rod, a thin film, a wire, a lattice or the like and used as a carrier which carries the porphyrin complex on the surface or the like, or the conductive material may be mixed as a powder material with the porphyrin complex and the resulting mixture may be used as a material to be molded by compression or the like. Particularly, by using a carrier made of a metal or a metal oxide, the mechanical strength of the CO₂ reduction catalyst or the CO₂ reduction electrode can be improved.

As the conductive material, a composite of a carbon material and a conductive resin, a conductive ion exchange resin or the like may also be used. Further, a resin material such as an ionomer may be used as well.

When the conductive material is a carrier, the carrier preferably comprises a porous part. Typically, it is preferred that the carrier be entirely porous; however, the carrier may be partially non-porous, in other words, the carrier may contain a compact part.

It is desired that the porous part have a pore distribution whose peak is in a range of 5 nm to 20 μm. By this pore distribution, the catalytic activity can be improved. The term “pore” used herein means a void observed in a cross-section of the porous part. Further, the term “pore distribution” means the distribution of pore sizes (void widths) per unit length on the porous outermost surface observed in a cross-section of the porous part. The pore distribution can be determined by, for example, measurement by a gas adsorption method, measurement by mercury intrusion porosimetry, particle size distribution measurement by a laser diffraction-scattering method, dry density measurement by a constant-volume expansion method, measurement using an AFM (atomic force microscope), or image processing of a TEM image.

In the present embodiment, the pore distribution of the carrier preferably has a plurality of peaks in the above-described range. By this, an increase in the surface area, an improvement in the diffusibility of ions and reactants and a high electroconductivity can all be realized at the same time.

A carrier having such a porous structure can be prepared by compressing a powder-form conductive material, or by etching a pore-free compact material and thereby forming pores.

The carrier may also have through-holes. Such a structure can also be formed by, for example, removing a part of the above-described carrier by etching or the like.

In this manner, by allowing the carrier to have a porous structure or providing the carrier with through-holes, the diffusibility of ions and reactants can be improved with pores and through-holes while maintaining a high electroconductivity and large active surface area of the catalyst. As the surface area of the catalyst is increased and the mass of reactants is thereby increased, the supply of products and raw material substances are limited by substance diffusion; however, problems associated therewith can also be solved by the porous structure or through-holes at the same time.

The catalyst according to one embodiment comprises a porphyrin complex layer on such a carrier.

The porphyrin complex layer may exist on a part of the carrier surface and is not required to cover the whole carrier. It is preferred that the carrier and this porphyrin complex layer have an electrical continuity with each other.

FIG. 1 is a conceptual drawing that schematically shows a CO₂ reduction catalyst 100 according to one embodiment. In the catalyst 100 shown in FIG. 1, a porphyrin complex layer 102 is laminated on a conductive material 101, and the conductive material 101 and the porphyrin complex layer 102 have an electrical continuity. The porphyrin complex layer may consist of only a porphyrin complex or have a structure in which a porphyrin complex is dissolved or dispersed in a solid conductive medium such as a conductive resin.

The structure of the CO₂ reduction electrode of this embodiment is not restricted to the one shown in FIG. 1, as long as the conductive material and the porphyrin complex layer are in an electrically continuous state. Further, in the CO₂ reduction electrode, the form of the conductive material is not particularly restricted and, for example, the conductive material may be take any form of a thin film, a lattice, particles and a wire.

In the catalyst of another embodiment, the conductive material and the porphyrin complex may be integrated. For example, the catalyst may be obtained by forming a single composition layer containing both the conductive material and the porphyrin complex on a substrate. Alternatively, the catalyst may be obtained by molding a single composition containing both the conductive material and the porphyrin complex into a plate shape, a rod shape or the like.

In yet another embodiment, the catalyst can be obtained by molding a mixture, in which a powder-form conductive material and a powder-form porphyrin complex are mixed, by compression or the like. In this case, a conductive resin or the like may also be used in combination as a binder.

These catalysts can each be directly used as a CO₂ reduction electrode; however, they can also each be arranged on a support made of a metal or the like and used as a CO₂ reduction electrode.

Further, it is preferred that the catalyst according to these embodiments contain a surfactant in the porphyrin complex layer or the like. The use of a surfactant makes the catalyst more likely to desorb a gas generated by a reduction reaction. As a result, a large contact area can be maintained between the catalyst and an electrolyte solution, so that the reduction reaction can be further promoted.

As the surfactant, for example, hydrophilic group-containing vinyl compounds such as polyvinylpyrrolidone and polyvinyl alcohols, derivatives thereof and polymers can be used. Other material may also be used as long as it has a function equivalent to that of the above-described compounds and the like.

Further, it is preferred that the catalysts according to these embodiments contain an ion exchange resin. By using an ion exchange resin such as Nafion (registered trademark), for example, adsorption of ions contributing to the reaction can be controlled. In addition, depending on the intended use, the type of the ion exchange resin is not restricted, and a material which has a function comparable to that of an ion exchange resin may be used as well.

FIG. 2 is a schematic drawing that shows one example of the structure of CO₂ reduction device 200 according to the present embodiment. The CO₂ reduction device 200 shown in FIG. 2 comprises: a container 201; a CO₂ reduction electrode 202; an oxidation electrode 203 which oxidizes water; a power supply element 204 which is electrically connected to the CO₂ reduction electrode 202 and the oxidation electrode 203; and an electrolyte solution 205 which is retained in the container 201 and is in contact with the CO₂ reduction electrode 202 and the oxidation electrode 203. The CO₂ reduction electrode 202 is the CO₂ reduction electrode according to the present embodiment and, as the CO₂ reduction electrode 202, the catalyst according to the present embodiment may be arranged on the surface of an electrode made of a metal or the like, or the catalyst according to the present embodiment can be used as is. The porphyrin complex layer of the CO₂ reduction electrode is required to be in contact with the electrolyte solution.

The electric power supplied by the power supply element 204 may be an electric power obtained from a system; an electric power obtained from conversion of kinetic energy, potential energy, thermal energy or the like into electrical energy; an electric power obtained from conversion of light energy by a solar cell or the like; an electric power obtained from conversion of chemical energy of a fuel cell, a storage battery or the like; or an electric power obtained from conversion of sound vibration or the like. In the present embodiment, the electric power is preferably one obtained from conversion of natural energy, particularly solar energy. In order to allow a reduction reaction by the catalyst to take place, it is required that the electric power be not smaller than the difference between the redox potential generated by oxidation of water and the CO₂ reduction potential, and an electromotive force of not less than 1.06 V is necessary for the conversion of CO₂ into methane while an electromotive force of 1.2 V is necessary for the conversion of CO₂ into methanol and an electromotive force of not less than 1.33 V is necessary for the conversion of CO₂ into CO. Therefore, the electromotive force is preferably not less than 1.0 V, more preferably a larger voltage to include an overvoltage, still more preferably not less than 1.3 V.

The electrolyte solution 205 is stored in, for example, a container such as an electrolyte solution tank. It is also possible to replenish the electrolyte solution 205 through a supply flow path. In this case, a heater and a temperature sensor may be arranged in a part of the supply flow path. Further, the inside of the container may be filled with vaporized components of the electrolyte solution 205.

The electrolyte solution 205 contains water (H₂O) and carbon dioxide (CO₂). Examples of the electrolyte solution 205 include aqueous solutions containing phosphate ions (PO₄ ²⁻), borate ions (BO₃ ³⁻), sodium ions (Na⁺), potassium ions (K⁺), calcium ions (Ca²⁺), lithium ions (Li⁺), cesium ions (Cs⁺), magnesium ions (Mg²⁺), chloride ions (Cl⁻), bicarbonate ions (HCO³⁻) and/or the like. For example, as the electrolyte solution 205, an aqueous solution containing LiHCO₃, NaHCO₃, KHCO₃, CsHCO₃ or the like can be used. The electrolyte solution 205 may also contain an alcohol such as methanol, ethanol or acetone. Further, different electrolyte solutions may be used as the electrolyte solution in which the oxidation electrode 203 is immersed and the electrolyte solution in which the CO₂ reduction electrode 202 is immersed. In this case, it is preferred that the electrolyte solution in which the oxidation electrode 203 is immersed contain at least water and the electrolyte solution in which the CO₂ reduction electrode 202 is immersed contain at least carbon dioxide. In addition, the production ratio of carbon compounds can be changed by adjusting the amount of water contained in the electrolyte solution in which the CO₂ reduction electrode 202 is immersed. Moreover, carbon dioxide may be blown into the electrolyte solution 205 by bubbling or the like. For this purpose, the device according to the present embodiment can be provided with a carbon dioxide introduction pipe 206.

As the electrolyte solution 205, an ionic liquid which contains a salt of a cation such as an imidazolium ion or a pyridinium ion and an anion such as BF₄ ⁻ or PF₆ ⁻ and is in a liquid state over a wide temperature range, or an aqueous solution thereof can be used. Examples of other electrolyte solution include amine solutions of ethanolamine, imidazole, pyridine or the like, and aqueous solutions thereof. Examples of the amine include primary amines, secondary amines and tertiary amines.

Examples of the primary amines include methylamine, ethylamine, propylamine, butylamine, pentylamine and hexylamine. The hydrocarbons of these amines may be substituted with an alcohol, a halogen or the like. Examples of an amine whose hydrocarbon is substituted include methanolamine, ethanolamine and chloromethylamine. The hydrocarbons may also contain an unsaturated bond. Such hydrocarbons are also applicable to secondary amines and tertiary amines.

Examples of the secondary amines include dimethylamine, diethylamine, dipropylamine, dibutylamine, dipentylamine, dihexylamine, dimethanolamine, diethanolamine and dipropanolamine. The substituted hydrocarbons may be different, and this is also applicable to tertiary amines. Examples of amines with different hydrocarbons include methylethylamine and methylpropylamine.

Examples of the tertiary amines include trimethylamine, triethylamine, tripropylamine, tributylamine, trihexylamine, trimethanolamine, triethanolamine, tripropanolamine, tributanolamine, tripropanolamine, trihexanolamine, methyldiethylamine and methyldipropylamine.

Examples of the cation in the ionic liquid include a 1-ethyl-3-methylimidazolium ion, a 1-methyl-3-propylimidazolium ion, a 1-butyl-3-methylimidazole ion, a 1-methyl-3-pentylimidazolium ion and a 1-hexyl-3-methylimidazolium ion.

These imidazolium ions may be substituted at the 2-position. Examples of a cation which is an imidazolium ion substituted at the 2-position include a 1-ethyl-2,3-dimethylimidazolium ion, a 1,2-dimethyl-3-propylimidazolium ion, a 1-butyl-2,3-dimethylimidazolium ion, 1,2-dimethyl-3-pentylimidazolium ion and a 1-hexyl-2,3-dimethylimidazolium ion.

Examples of the pyridinium ion include methylpyridinium, ethylpyridinium, propylpyridinium, butylpyridinium, pentylpyridinium and hexylpyridinium ions. These imidazolium ions and pyridinium ions may be substituted at an alkyl group and may contain an unsaturated bond.

Examples of the anion include a fluoride ion, a chloride ion, a bromide ion, an iodide ion, BF₄ ⁻, PF₆ ⁻, CF₃COO⁻, CF₃SO₃ ⁻, NO₃ ⁻, SCN⁻, (CF₃SO₂)₃C⁻, a bis(trifluoromethoxysulfonyl)imide anion and a bis(perfluoroethylsulfonyl)imide anion. A dipolar ion in which a cation and an anion of an ionic liquid are bound via a hydrocarbon may also be used.

The pH of the electrolyte solution in which the CO₂ reduction electrode 202 is immersed is preferably lower than the pH of the electrolyte solution in which the oxidation electrode is immersed. This allows hydrogen ions, hydroxide ions and the like to move easily. In addition, a liquid junction potential based on the pH difference can be effectively used in redox reaction.

The electrolyte solution in which the electrode 202 is immersed and the electrolyte solution in which the oxidation electrode 203 is immersed can be separated using an ion exchange membrane. The ion exchange membrane has a function of allowing some of the ions contained in the electrolyte solutions in which each electrode is immersed to permeate therethrough, that is, a function of blocking one or more ions contained in either of the electrolyte solutions. As a result, for example, a difference in pH or ionic strength can be created between the two electrolyte solutions. By such a constitution, the CO₂ reduction reaction can be promoted.

Examples of the ion exchange membrane include cation exchange membranes such as Nafion (registered trademark) and Flemion (registered trademark), and anion exchange membranes such as Neosepta (registered trademark) and Selemion (registered trademark). A bipolar membrane in which cation exchange membrane and anion exchange membrane are layered such as Neosepta (registered trademark) can be used. The bipolar membrane is preferably employed if the difference of pH between the electrolyte solution in which the anode is immersed and the electrolyte solution in which the cathode is immersed is large, for example, the alkaline electrolyte and the acidic electrolyte are used. In cases where the movement of ions between the two electrolyte solutions does not have to be controlled, it is not necessary to arrange such an ion exchange membrane.

The CO₂ reduction catalyst can be recycled from a degraded state by cleaning based on electrochemical redox, a treatment with a compound having a cleaning effect, or cleaning with heat, light or the like. It is preferred that the CO₂ reduction device be made usable for or tolerable such recycle of the catalyst by adjusting the voltage applied to the electrodes. It is also preferred that the CO₂ reduction device according to the present embodiment have such a function of recycling the catalyst.

In order to accelerate the supply of ions or substances to the electrode surface, the CO₂ reduction device may further comprise a stirrer.

Further, the CO₂ reduction device may also comprise a measuring instrument(s) such as a thermometer, a pH sensor, a conductivity meter, an electrolyte solution analyzer and a gas analyzer, and it is preferred that parameters in the CO₂ reduction device be measured by these measuring instruments and the CO₂ reduction device be controllable based on the measured values.

The CO₂ reduction device may be a batch-type reactor or a flow-type reactor. In the case of a flow-type reactor, it is desired that a supply flow path and a discharge flow path of an electrolyte solution be secured. If the batch-type reactor is employed, it is preferred that the reactor is operated when surplus power is generated and is stopped when the power demand is large and surplus power is not generated.

Next, an operation example of the CO₂ reduction device will be described. Here, as an example, a case of producing carbon monoxide from carbon dioxide using an iron (III) chloride complex as the porphyrin complex will be described.

First, electrons gather on the CO₂ reduction electrode from the power supply element, and the Fe ion contained in the porphyrin complex is reduced. CO₂ dissolving in the electrolyte solution is coordinated with this Fe ion. Meanwhile, a CO₂ reduction reaction consequently takes place as represented by the following Formula (1), wherein CO₂ reacts with hydrogen ions to generate carbon monoxide, which is a carbon compound, and water (hydroxide ions when the electrolyte solution is alkaline). The thus generated carbon monoxide dissolves in the electrolyte solution at an arbitrary ratio. The area of the part where the CO₂ reduction reaction takes place is larger in a CO₂ reduction electrode having a porous structure than in a CO₂ reduction electrode having a non-porous structure. A recovery flow path may also be arranged in the container of the electrolyte solution so as to recover the generated carbon compound therethrough.

2CO₂+4H⁺+4e⁻→2CO+2H₂O

(2CO₂+2H₂O+4e⁻→2CO+4OH⁻)   Formula (1)

Meanwhile, on the oxidation electrode, an oxidation reaction of water takes place as represented by the following Formula (2), whereby oxygen and hydrogen ions (water when the electrolyte solution is alkaline) are generated and electrons flow to the power supply element.

2H₂O→4H⁺+O₂+4e⁻

(4OH⁻→2H₂O+O₂+4e⁻)   Formula (2)

The hydrogen ions (water) generated by the oxidation reaction migrate to the CO₂ reduction electrode.

The embodiments described herein are presented for the illustration purpose only, and the scope of the present invention is not restricted thereto.

EXAMPLES Example 1 Production Example of CO₂ Reduction Electrode

In a 200-ml flask, 5,10,15,20-tetraphenyl-21H,23H-porphyrin iron (III) chloride was dissolved in chloroform, and Ketjen Black was added thereto. Subsequently, the solvent was removed using an evaporator, and the complex was allowed to adsorb to Ketjen Black.

This Ketjen Black was added to a 2.5-wt % Nafion solution and dispersed by ultrasonication, and the resulting dispersion was spray-coated on a carbon paper (GDL10BA) to prepare a CO₂ reduction electrode.

Comparative Example 1

Ketjen Black carrying 30 wt % of gold thereon was added to a 2.5-wt % Nafion solution and dispersed by ultrasonication, and the resulting dispersion was spray-coated on a carbon paper (GDL10BA) to prepare an electrode.

Example 2

After adding 15 ml of ethanol to 1.5 g of Ketjen Black, 500 μl of triethoxy-3-(2-imidazolin-1-yl)propyl silane was further added. Then, 2.5 mL of pure water was added thereto, and the resultant was allowed to react at 60° C. for 1 hour. After the reaction, the resulting solution was filtered, washed with ethanol and water, and the dried under reduced pressure.

A porphyrin complex was added to the thus obtained Ketjen Black in an amount of 36 wt %, and the resultant was dissolved in chloroform. After removing the solvent using an evaporator, the complex was allowed to adsorb to Ketjen Black.

This Ketjen Black was added to a 2.5-wt % Nafion solution and dispersed by ultrasonication, and then resulting dispersion was spray-coated on a carbon paper (GDL10BA) to prepare an imidazoline-modified CO₂ reduction electrode.

Comparative Example 2

After adding 15 ml of ethanol to 1.5 g of the gold-carrying Ketjen Black used in Comparative Example 1, 500 μl of triethoxy-3-(2-imidazolin-1-yl)propyl silane was further added. Then, 2.5 mL of pure water was added thereto, and the resultant was allowed to react at 60° C. for 1 hour. After the reaction, the resulting solution was filtered, washed with ethanol and water, and then dried under reduced pressure.

This Ketjen Black was added to a 2.5-wt % Nafion solution and dispersed by ultrasonication, and then resulting dispersion was spray-coated on a carbon paper (GDL10BA) to prepare an imidazoline-modified gold catalyst electrode.

The thus prepared electrodes were each measured under the following conditions using an electrochemical analyzer.

The resistance component was determined by measuring the impedance under the following conditions: 0.95 V vs Ag/AgCl, amplitude=10 my, frequency=100 to 0.1 Hz. In linear sweep voltammetry (LSV), an H-type cell was used at 1 mV/sec in a range of 0 to −0.6 V along with Selemion (registered trademark) as an electrolyte membrane. The measurement was performed using a platinum foil as a counter electrode, an Ag/AgCl electrode as a reference electrode, and a 0.5 M aqueous K₂CO₃ solution as an electrolyte solution.

Comparative Example 3

Further, as a control, an electrode prepared in accordance with Y. Tian, et al., The Journal of Physical Chemistry B vol. 110. pp. 23478 (2006) was used. Specifically, an electrode which was prepared by immersing a conductive material in an aqueous solution in which perchloric acid (0.1 M) and chloroauric acid (4 mM) were dissolved and applying a voltage of −0.08 V to an Ag/AgCl (saturated KCl) reference electrode was used.

[Evaluation] [Electrochemical Measurement]

When the electrode obtained in Example 1 was evaluated at a size of 4-cm square, a reduction current of 2.5 mA/cm² was observed at a constant voltage of −0.6 V (vs RHE), and the Faraday efficiency of CO in this case was found to be 50%. Further, for the electrode of Example 2, a reduction current of 1 mA/cm² was observed, and the Faraday efficiency of CO in this case was found to be 50%.

Meanwhile, for the electrode of Comparative Example 1, a reduction current of 4.7 mA/cm² was observed, and the Faraday efficiency of CO in this case was found to be 35%. For the electrode of Comparative Example 2, a reduction current of 4.2 mA/cm² was observed, and the Faraday efficiency of CO in this case was found to be 30%.

Each electrode was also evaluated in the same manner at a size of 1-cm square using a 0.25 M aqueous K₂CO₃ solution as an electrolyte solution.

For the electrode obtained in Example 1, reduction currents of 4.2, 2.0 and 0.8 mA/cm² were observed at constant voltages of −0.6, −0.5 and −0.4 V (vs RHE), respectively, and the Faraday efficiency of CO was found to be 53, 60 and 45% in these cases, respectively. For the electrode of Comparative Example 3, a reduction current of 2.0 mA/cm² was observed at a constant voltage of −0.5 V (vs RHE), and the Faraday efficiency in this case was found to be 55%.

It was found that the catalysts of Examples have a high CO selectivity and a performance equivalent or superior to those of Comparative Examples.

From these results, it is seen that, as compared to conventional electrodes using a noble metal, the CO₂ reduction electrode according to the present embodiment can be produced at a lower cost and has an equivalent or superior performance.

While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fail within the scope and spirit of the invention. 

1. A CO₂ reduction catalyst comprising: a conductive material; and a porphyrin complex which is carried on said conductive material and represented by the following Formula (A):

wherein, each R independently represents a group selected from the group consisting of hydrogen, hydrocarbon groups having 1 to 12 carbon atoms, a hydroxyl group, an amino group, a carboxy group, a sulfo group, a mercapto group and a formyl group and are optionally the same or different, and adjacent R groups are optionally bound with each other via a hydrocarbon chain having 1 to 12 carbon atoms to form a cyclic structure; M represents a (2+n)-valent metal ion, wherein n is a number of 0 or larger; X represents an n-valent anion; and the total number of hydroxyl groups, amino groups, sulfo groups and mercapto groups that are contained in one molecule of said porphyrin complex is 10 or less.
 2. The CO₂ reduction catalyst according to claim 1, wherein said metal ion is Fe ion.
 3. The CO₂ reduction catalyst according to claim 1, wherein said total number of hydroxyl groups, amino groups, sulfo groups and mercapto groups that are contained in one molecule of said porphyrin complex is 8 or less.
 4. The CO₂ reduction catalyst according to claim 1, wherein said porphyrin complex is represented by the following Formula (A-1) or (A-2):

wherein, each R independently represents a group selected from the group consisting of hydrogen, hydrocarbon groups having 1 to 12 carbon atoms and are optionally the same or different, and adjacent R groups are optionally bound with each other via a hydrocarbon chain having 1 to 12 carbon atoms to form a cyclic structure; and said hydrocarbon groups and hydrocarbon chain are optionally substituted with a hydroxyl group, an amino group, a carboxy group, a sulfo group, a mercapto group or a formyl group, with a proviso that the total number of hydroxyl groups, amino groups, sulfo groups and mercapto groups that are contained in one molecule of said porphyrin complex is 10 or less.
 5. The CO₂ reduction catalyst according to claim 1, further comprising a surfactant.
 6. A method of reducing carbon dioxide, said method comprising: bringing a CO₂ reduction electrode comprising the CO₂ reduction catalyst according to claim 1 into contact with an electrolyte solution; and introducing carbon dioxide to said electrolyte solution and reducing the thus introduced carbon dioxide by said electrode.
 7. A CO₂ reduction device comprising: an oxidation electrode; a CO₂ reduction electrode comprising the CO₂ reduction catalyst according to claim 1; and a power supply element connected to said oxidation electrode and said CO₂ reduction electrode.
 8. The device according to claim 7, wherein said power supply element comprises a semiconductor layer which performs charge separation with light energy. 